An electrochromic material undergoes a reversible color change upon the adsorption and desorption of small cations. This property can be exploited to fabricate a device that changes color upon the application of a voltage potential.
The typical electrochromic device comprises an electrochromic layer and an ion storage layer sandwiched between two conducting substrates, at least one of which is transparent. Optionally, the electrochromic layer and the ion storage layer can be separated by an ion-conducting electrolyte layer. Optical properties of the electrochromic device change when ions (e.g., hydrogen ions or lithium ions) intercalated within the structure of the ion-storage layer are removed and interposed within the structure of the electrochromic material in response to an electric potential applied to the conductive substrates. Ions are removed and returned to the ion-storage layer by reversing the polarity of the applied potential, thereby returning the electrochromic device to its original optical state.
The electrochromic layer and the ion storage layer are similar in that they both adsorb and desorb mobile ions in response to an applied electric field. A simple model for understanding electrochromic devices assumes that the electrochromic layer colors and clears during ion adsorption/desorption, while the ion storage layer remains transparent. However, practical electrochromic devices can be made if the ion storage layer colors as well. For example, if the electrochromic layer cycles from clear to blue on desorption (e.g. iron(III) ferrocyanide, or insoluble Prussian Blue), and the ion storage layer cycles from clear to blue upon ion adsorption (e.g. tungsten oxide), the overall device will cycle from clear to blue. If the electrochromic layer cycles from clear to blue on desorption (e.g. iron(III) ferrocyanide), and the ion storage layer cycles from clear to yellow upon ion adsorption (e.g. vanadium oxide), the overall device will cycle from clear to green. Numerous combinations are possible.
Furthermore, if the ion-conducting electrolyte layer is opaque and the electrochromic layer cycles from clear to blue, the entire device will cycle from blue to the color of the ion-conducting electrolyte layer, regardless of the coloration of the ion storage layer.
The construction of an electrochromic device typically involves coating electrochromic material onto a transparent, conductive substrate. If the electrochromic material is a metal oxide, there are several proven coating methods available. These include evaporation deposition, electro-deposition, coating a metal alkoxide from an alcoholic solution and heating in excess of 200° C., and generating the metal oxide in situ within a polymer composite. If the electrochromic material is iron (III) ferrocyanide or its analogs, deposition methods are more limited. These include electroless deposition and electro-deposition. The prior art describes general methods of casting an electrochromic polymer composite that require the incorporation of both electrochromic and conductive particles. Related technology is the dispersion of pigments in water-based paints.
An electrochromic device comprising flexible plastic substrates, such as polyethylene terephthalate coated with indium tin oxide, would have advantages over rigid electrochromic devices comprising glass substrates. These advantages include light weight, durability, shapability and low cost. Of particular interest is a flexible electrochromic device incorporating metal ferrocyanides in the electrochromic layer, as these devices demonstrate strong color contrast and good durability. However, existing methods of coating metal ferrocyanides on conductive plastic substrates require improvement. These deposition methods are not compatible with high-speed roll-to-roll processing, and in the case of electrodeposition, tend to generate considerable amounts of toxic waste solutions from spent plating baths.
Glass electrochromic devices have not achieved broad commercial acceptance in architectural, automotive or eyewear applications, due to practical limitations. First, glass electrochromic devices can be prohibitively expensive to manufacture. Second, glass electrochromic devices cannot function for the decades required for architectural and automotive applications. With each cycle, an electrochromic device suffers a minute but cumulative deterioration in performance, due to the accumulation of an irreversible colored “bronze” and trapped gas. Third, glass electrochromic devices are too heavy for eyewear applications and can also shatter to dangerous shards upon impact of a foreign object.
Plastic electrochromic devices address these limitations. Manufacturing costs are controlled by low capital requirements and high throughput. For example, as described below in accordance with the present invention, an electrochromic layer or an ion storage layer could be coated on a continuous wide web of electrically conductive polyethylene terephthalate film at a rapid rate. The layers could then be laminated together using an adhesive ion conducting electrolyte. If this laminated film is applied to architectural and automotive glazing, the film could be replaced if its performance deteriorates over time. If the laminated film is applied to polycarbonate, or if the electrochromic device is coated on the polycarbonate directly, the electrochromic device would be light and safe enough for eyewear.